1. Field of the Invention
This invention relates to a reactance measuring means which in one embodiment is a capacitance measuring means which provides an output suited to a reactance type pressure sensor, which sensor may be located non-adjacent to the signal conditioning and transmitter electronics which electronics control the current in two wires to be representative of the reactance of the variable reactance means.
2. Description of the Prior Art
Capacitance sensing circuitry which provides a DC signal proportional to capacitance has been described in U.S. Pat. Nos. 3,271,669, 3,318,153, 3,646,538, 3,854,039 and 3,975,719. Circuitry which provides control of direct current through a transmitter as a function of a DC signal is described in U.S. Pat. Nos. 3,859,594 and 3,680,384. Many industrial process indicators and controllers are in use which are scaled to respond to current from a DC two wire transmitter, so it has long been desirable to have a transmitter which in addition to converting a reactance sensor output such as from the capacitance pressure sensor or capacitance level sensor to control a direct current supplied to the transmitter, to also provide a variable reactance sensor remote from, or non-adjacent to, the signal conditioning and transmitter electronics, which sensor may be in a hostile environment, and which provides access to the signal conditioning and transmitter electronics without human exposure to such environment.